Over 300 million scrap tires are generated a year in North America. These scrap tires take up large amounts of space in landfills or are illegally dumped, providing breeding grounds for mosquitoes and rodents. Large stockpiles of scrap tires also have the potential for fires that are detrimental to the environment and extremely hard to extinguish.
Fortunately, markets now exist for most of the scrap tires generated each year. The major market for scrap tires, at some 47% of all recovered used tires, is tire derived fuels, which in 2009 consumed some 2,084 thousand tons of all recovered tires. The second largest single market for ground tire rubber (“GTR”), utilizing significantly smaller mesh sizes than that found in tire derived fuels, consumed approximately 31% of the available scrap tires, or some 1,350 thousand tons of all recovered tires per year in 2009. This GTR is incorporated into bituminous (e.g., asphalt) paving material as a cost reduction and performance enhancement additive that consumes over 18 million tires a year.
Approximately 94% of all roads in America are paved and all states are aggressively searching for ways to lower material costs and improve asphalt performance. With twenty one states currently using rubber modified asphalt, GTR containing asphalt is projected to eclipse landscape cover as the number one scrap tire consuming market within the next several years.
Blending GTR with asphalt can produce a pavement with a number of advantages such as longer lasting road surfaces, reduced road maintenance due to lesser cracking and rutting, reduced pavement thickness, lower road noise, and shorter breaking distances. According to the Rubber Pavements Association, a 2.5 cm thick overlay of hot-mix asphalt will consume about 2,000 tires per lane mile with seal coats consuming about 500 tires per lane mile when spray applied. Therefore, using rubber-containing asphalt is an effective way to reduce the problematic stockpiles of scrap tires.
Blending rubber with asphalt can, however, be challenging. Rubber particles are generally insoluble in asphalt and can settle out of the asphalt mixture during storage or transport. Also, blending is often done at high temperatures, which swells the rubber and increases the viscosity of the binder. Reheating or prolonged heating during storage tends to devulcanize the rubber, physically degrading the overall performance properties of the rubber particulate. It can also eliminate the particulate nature of the GTR and lead to loss of the desirable improvements in physical properties and lower costs that can be attained by the use of GTR enhanced asphalt. These phenomena limit the opportunities for using rubber-containing asphalt mixtures, requiring that such mixtures be used within a short period after their manufacture and in the vicinity of the manufacturing facility.
Several methods have been tried to improve the storage and transport stability of rubber-containing asphalt mixtures. For example, light hydrocarbon solvents or highly aromatic, high-boiling mineral oil have been added to such mixtures. These methods are, however, costly and of questionable environmental impact. Other methods devulcanize the rubber with high temperatures and/or adding oxidizers or devulcanization agents. However, such methods are capital intensive and therefore costly, require additional steps, and can use reactive agents. Another strategy for resolving these issues has been to use mobile mixing units that mix the GTR and asphalt on site. Such units are expensive and they may not routinely be available.
It is thus desirable to improve the processes of incorporating rubber into bituminous materials like asphalt. One step processes, implemented at the point of manufacture, that can improve the stability of such mixtures and reduce separation can allow longer storage and transport times. Mixtures with lower costs and greater stability have increased opportunities for use, which ultimately means that the benefits of using these materials can be realized in more locations and that there can be further reductions in the number of scrap tires in landfills.